In the manufacture of boats, airplanes and other structures where woven fibrous materials become pan of the composite skin or coveting material, the economic creation of a smooth finished surface has been a ubiquitous problem. Almost by definition, the woven nature of an outer ply of the composite material presents a rough and porus surface, the preparation of which is expensive and time-consuming before it is possible to apply the final coating, such as paint, especially when an aerodynamic finish is required.
The prior an has adopted several approaches to solving this problem and many of the solutions have seen use with fiber glass structures, especially in the automotive and boat industries.
One popular method of smoothing the irregularities of the woven fabric involves the use of a liquid gel-coat, sprayed onto the tool, prior to the laying down of the first layer of woven fabric. The gel-coat transfers into the woven material during processing to create a filler. The difficulty with this process however is the control of thickness and other aspects of a uniform application of the gel-coat, requiring later finishing steps, such as sanding, to rid the surface of undulations and irregularities. Another disadvantage is the added weight of the gel-coat material, a significant penalty in the aircraft industry.
Another method uses dry sheets of polyester, placed on the tool prior to application of the structural woven fiber plies. This method has not proved satisfactory because the sheets wrinkle and are difficult to control and keep in place while laying up the outer ply of the composite structure, especially when used on a tool having complex shapes.
Adhesion of smoothing materials to the woven fiber surface is also unsatisfactory and expensive. In some such systems the adhesion materials are different than the epoxy which impregnates the structural plies of woven material, creating a reaction between the two materials which alters the structural characteristics of the composite, which is a disagreeable result.
Accordingly, the primary object of the present invention is to provide a means and method of smoothing the outer ply of a woven composite structure so as to eliminate costly preparation of the surface for final finish.
More specifically, the object of the invention is to provide a smooth outer composite material surface by using a random oriented aramid, polyester, graphite or fiberglass resin impregnated mat as the outer ply for composite resin lay ups.
Another object of the invention is to provide a means for smoothing the outside surface of a woven fabric composite lay up without the additional step, either in the tool or after the molding process, of including or adding to the composite an independent or separate layer of material for smoothing purposes, but instead to unite the smoothing material with the outer ply so that the smoothing agent and the outer ply are applied together.
An additional objective of the invention is to utilize the benefits of a structurally weak non-woven material as a smoothing agent, but to combine the woven and nonwoven materials so that the weaker material may be handled and applied to a tool integrally with the stronger woven material, thus protecting the physical integrity of the non-woven material.
A still further object of the invention is to provide a smoothing agent for preimpregnated woven sheets of material which can be applied with the same epoxy as that which is used to impregnate the woven sheet, thus eliminating the problems inherent in the prior art's juxtapositioning of dissimilar epoxies.